Partial hydrogenation of FAMEs with high content of C18:2 dienes. Selective hydrogenation of tobacco seed oil-derived biodiesel

The study reports on Pd(5%)/C-catalyzed partial hydrogenation of two C18:2-rich FAME mixtures (66.9 and 75.1 mol% C18:2) respectively prepared by esterification of technical linoleic acid and transesterification of tobacco seed oil. The mixtures were taken as representatives of C18:2-rich FAMEs obtainable from a variety of non-edible oils, which have potential as alternative feedstocks for the development of 2nd-generation biodiesel. In n-heptane, under mild conditions (15 °C; 0.1 MPa H2; 45–60 min; Pd/C, 2.1–5.7 wt%), the FAME mixtures were converted to C18:1 with high yield (83–92%) and selectivity (93–95%). Stearate abundance in the hydrogenated mixture was kept in acceptable limits (5.5–8.1%); E-C18:1 formation and C[dbnd]C scattering were respectively in the ranges 20.3–34.8% and 15–24%. At 0 °C, under moderate H2 pressure (1 MPa), the formation of trans-C18:1 monoenes and C[dbnd]C scrambling were less pronounced and respectively equal to 9.0% and 7% after 45 min, using a lower catalyst load (1.0 wt%): under the latter conditions the C18:2-component was markedly curtailed (66.9 → 13.6%) in the final product, while stearate abundance was still quite modest (9.3%). Geometric and positional isomerization increased over time with the progress of C18:2 conversion and were favored by higher temperature and catalyst load. Under solventless conditions hydrogenation was slower than in n-heptane; moreover, both cis → trans isomerization and C[dbnd]C bond scattering were larger than in the hydrocarbon solvent. Remarkably, Pd/C can act as a versatile catalyst, being able to effectively promote the conversion of the FAME mixtures to stearate under uncommon very mild conditions.